The invention relates to a releasable locking device for a vehicle steering column adjustable in the longitudinal direction, having a positionally fixed steering column part and a steering column section displaceable relative to the latter, having at the end a steering wheel, the fixing of which can be performed by a hydraulic piston-cylinder unit which is arranged axially parallel to the steering column and is coupled between a positionally fixed steering column part and a displaceable steering column section and which can be controlled by a mechanical actuating member.
PTC Patent Document WO 80/01478 describes a locking device for an adjustable vehicle steering column in which a piston-cylinder unit is coupled by in each case one end to two steering column parts which, when this piston-cylinder unit is pressurized, can be pushed apart. The piston-cylinder unit is actuated by the oil flow from a feed line and controlled via an operator-controllable valve. In the case of these pressure connections, which are necessary at both ends, the oil pressure must be maintained even after the locking position of the steering column has been reached.
A locking device which comprises a hydraulically pressurized piston is known from a device for steering-wheel adjustment in motor vehicles and described in German Utility Model 8,812,673. Here, an otherwise unpressurized piston is pressurized via a hydraulic fluid for the purpose of releasing clamping of the steering column, the piston thereby acting against the clamping force of Belleville springs. The clamping force for locking the displaceable steering column part is here produced exclusively by the Belleville springs, which clamp two half-shells of a guiding body to the steering column part. It is impossible for displacement-proof fixing of the steering column in the case of an impact on the steering wheel to result from this non-rigid, frictional clamping. If the clamping of the steering column is released by actuation of the mechanical actuating lever and by the pressure brought about thereby of the hydraulically pressurized piston on the Belleville springs, the steering column is freely displaceable in the guiding body and, by reason of its weight and that of the steering Wheel, will tend to slide downwards unbraked in the guiding body.
A piston-cylinder unit in a safety steering column which absorbs the impact energy coming into effect in the event of a collision is known in German Laid Open Application No. 2,417,543. For this purpose, the steering part is guided in axially displaceable fashion in a chamber in the steering column, said chamber containing an incompressible medium. Connected downstream of this chamber, via a flow passage, is a further chamber in which is arranged a controllable flow valve.
An object on which the invention is based is to design a locking device for a vehicle steering column vertically adjustable in the longitudinal direction in such a way that it is stable under impact and favorable in terms of operation.
The object is achieved according to the invention by providing an arrangement wherein the locking device has a sealed piston-cylinder unit which, with the cylinder chamber completely filled and subjected to pressure in the case of an impact on the steering wheel, can be locked in its various supporting positions by a blocking device arranged in the overflow path between the two cylinder chambers.
After the opening of the blocking device, the sealed hydraulic piston-cylinder unit arranged axially parallel to the steering column permits a stepless adjustment of the displaceable steering column section while, with the passage through its hydraulically closed system with the cylinder closed, it is completely rigid. This leads to the steering column in this zone consistently passing the impact forces which act on the steering wheel in the support structure. Complete filling with fluid of the cylinder chamber subjected to pressure in the case of an impact on the steering wheel can be achieved in a simple manner if both cylinder chambers correspond in the volume of fluid displaced and accepted, as results, for example, in the case of a piston rod on both sides. A rebound energy against a person bracing himself against the steering wheel in the case of an accident, such as that which could be produced, for example, in the case of a gas-filled spring device used for positioning the steering column, is not to be expected at all. The adjustment of the steering wheel in the axial direction of the steering column is thus made possible in a simple fashion mechanically by the actuation of the blocking device, but the displaceable steering column section does not sink downwards due to the damping caused by the limited cross-section of flow of a valve but can be pulled upwards or pressed downwards with slight resistance by the fluid flowing through the valve opening. A limitation of the displacement movement of the steering column section can here be brought about by providing limitation of the piston displacement in the cylinder.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.